slide when placed on the side of the glass, and are thus held firmly in position. Fig. 1 shows the position of the holes on the backs of the rubbers. The latter plan is the best for fastening the rubbers, as it allows them to be removed at any time for warming (a very essential point) or spreading fresh amalgam on them. Fig. 2 shows the position of the plate and rubbers when in their places.

R R Rubbers.

Fig. 5.—Sectional Diagram Showing Position of Collectors and Plate.

We now have the means for procuring electricity, but the method of collecting it has yet to be provided. To make this a conductor must be formed by cutting a piece of wood to the shape of Fig. 3. It should be about 6 inches from end to end, and must be carefully rounded so that no projections are left on it. It must then be covered carefully with tinfoil (which can be obtained from a chemist), the tinfoil being glued down as smoothly as possible. From the end of this conductor a piece of brass rod should be fixed, shaped as shown in Fig. 4. A piece about 12 inches long will be wanted. This must be bent at the ends, so that when the conductor is mounted on a stand consisting of a piece of glass rod 6½ inches high, fixed to the center of the stand (that is 5 inches from the opposite end to that at which the supports are), the glass plate revolves between two surfaces of the brass rod. Fig. 5 explains the arrangement, which is somewhat complicated to describe. The glass rod should be about 7 inches long, to allow of half an inch being inserted into a hole in the center of the conductor, which is thus supported 6½ inches high from the stand.

Fig. 6.—The Machine When Finished.

It now only remains to fasten several small pieces of brass wire about a quarter of an inch long, filed to a point, to the sides of the rod nearest the glass plate, as shown in Fig. 6, so that the plate revolves between a double row of points, which can be done with solder, and the machine is complete. The conductor can further be improved by inserting at the opposite end a small piece of brass rod two and a half inches high, surmounted by a brass ball, which is useful in some experiments. Care must be taken that the tinfoil of the conductor overlaps the brass rod at either end, and thus forms a metallic connection. If this is not done the conductor will not become charged sufficiently. If the conductor can be made of brass it will work better still, as a metallic connection is then insured. The conductor can be fastened to the glass rod on which it is supported by “Prout’s elastic glue,” or other cement, a hole being made in the center of the bottom of the conductor, and another in the stand of the machine for opposite ends of the glass rod.

The machine having been constructed, a few words will be useful in how to work it. Warmth and dryness are, above all things, essential. If the air of the room is damp it will be nearly impossible to obtain any result. Before working, the glass plate must be thoroughly warmed, taking care not to crack it, by being placed endwise before a good fire. A silk handkerchief is a useful adjunct to the machine.

The glass plate should be wiped quite free from dirt, and the glass support of the conductor must also be wiped, the handkerchief being made very hot. The rubbers must be taken off (if constructed so as to be movable, as described), and placed before the fire till quite hot. Their powers may be enormously increased by covering them with amalgam, as sold in the electrical shops, but a far better plan is to cover the cushions with tinfoil, which can be glued right round the rubbers and over the backs. This will need renewing at intervals, as the plate in turning wears it out.

Now, when the rubbers are quite hot and all the glass of the machine is dry and hot (this is necessary, because, if damp, the electricity would escape without producing any effect), the rubbers are put into their proper places on each side of the glass, and on turning the handle (which will be rendered easier if the machine is firmly clamped to the table) and approaching the knuckle to the conductor, a succession of brilliant sparks will be emitted from the conductor. If this does not happen either the glass or some part of the machine is damp, or the machine is not put together quite correctly, and must be examined to find out the fault.

A machine of the size described should give a spark an inch long when working properly. A great number of experiments may be performed with this machine with apparatus capable of being made at home. I give a final illustration (Fig. 6) to show how the machine looks when completed.

How to Make an Induction Coil.

To most boys electricity offers many attractions, and as I have recently constructed an induction coil out of materials which are cheap and easily obtained, I think I shall confer a benefit on many readers if I give them a short description of how this was accomplished, so that if like-minded they can proceed in the same way. Induction coils may be used for medical and scientific purposes as well as for amusement, so that a good deal of work comes within their scope. An “induction coil” is composed principally of two portions—one is the “primary” coil, the other the “secondary.” It is the secondary coil that gives the spark, and on the length of this depends the power of the coil; in some instruments for scientific purposes it is composed of a wire nearly 300 miles long—but we are not going to soar to such heights as that!

To make the coil itself you want an ounce of “No. 24” cotton-covered wire, and two or three ounces of “No. 36.” This can be bought from an electrical supply dealer. If you are very ambitious, silk-covered wire can be used; this gives better effect, the insulation being more complete.

Fig. 1.—Front Disc.

AA, holes for primary wire. C, hole for core.

Fig. 2.—Back Disc.

BB, holes for secondary wire. C, hole for core.

To form the groundwork of the apparatus take a piece of mahogany about half an inch in thickness and polish it up to look ornamental; it should be about 4 inches by 6 inches for the sized coil I am describing. We now take another piece of mahogany about ¼ inch thick, and from it cut two circular pieces about 1½ inch in circumference; these are to form the ends of the coil; they must each have a hole ⅜ inch in diameter drilled in the center for the ends of the core to pass through. In one of them, which is to form the coil, two much smaller holes are drilled with a small bradawl to allow the ends of the primary coil to pass through (Fig. 1); in the other two similar holes are drilled further from the center for the ends of the secondary coil (Fig. 2). This having been done, we proceed